1. Field of the Invention
The present invention relates to a method and apparatus to multiplex a plurality of bitstreams, and more particularly, to a method and apparatus to multiplex bitstreams that are coded to have different frame lengths using asynchronous time alignment.
2. Description of the Related Art
As illustrated in FIG. 1, a multiplexing unit 120 multiplexes bitstreams coded by an A coder 100 and a B coder 110 that use different coding methods and outputs a stream. For example, the A coder 100 codes an audio signal using Advanced Audio Coding (AAC), the B coder 110 codes an audio signal corresponding to a multi-channel using Moving Picture Experts Group (MPEG) Surround, and the multiplexing unit 120 multiplexes the audio signals coded using different coding methods.
When the A coder 100 and the B coder 110 codes the bitstreams using different coding methods, the multiplexing unit 120 may use synchronous time alignment in which the length of each frame of the bitstream coded by the A coder 100 may be the same as that of the bitstream coded by the B coder 110, as illustrated in FIG. 2 or use asynchronous time alignment in which the frame length of the bitstream coded by the A coder 100 may be different from that of the bitstream coded by the B coder 110, as illustrated in FIG. 3.
When the multiplexing unit 120 multiplexes the coded bitstreams using asynchronous time alignment, it selects one of the A coder 100 and the B coder 110, and divides the bitstream coded by the selected coder on a frame basis and divides the bitstream coded by the non-selected coder based on a predetermined criterion for multiplexing. Since a decoding end cannot recognize information related to the size of data obtained by dividing the bitstream coded by the non-selected coder, the multiplexing unit 120 inserts length information of data obtained by dividing the bitstream coded by the non-selected coder. For example, when the multiplexing unit 120 generates a stream by multiplexing the bitstreams that are coded to have different frame lengths by the A coder 100 and the B coder 110, as illustrated in FIG. 4, it divides a bitstream corresponding to an (m−1)th frame 300 coded by the A coder 100 into a bitstream corresponding to a first (m−1)th frame 303 and a bitstream corresponding to a second (m−1)th frame 306 and inserts (n−2)th length information 330 for the bitstream corresponding to the first (m−1)th frame 303 and (n−1)th length information 335 for the bitstream corresponding to the second (m−1)th frame 306 in order to enable a decoding end to recognize the length of the bitstream corresponding to the first (m−1)th frame 303 and the length of the bitstream corresponding to the second (m−1)th frame 306.
However, when length information of each bitstream divided in a coding end is inserted and multiplexed, length information has to be generated for every bitstream and has to be inserted into a stream for transmission to the decoding end, making a coding process complex and time-consuming and increasing the size of data to be transmitted from the encoder to the decoding end.
If a frame length with respect to the B coder 110 is much different from a frame length with respect to the A coder 100 as illustrated in FIG. 5, a bitstream corresponding to a unit frame coded by the A coder 100 is divided into numerous bitstreams. As a result, the number of pieces of length information for the divided bitstreams that are to be inserted and the size of data that is to be inserted excessively increases.